In Long Term Evolution (LTE) wireless networks developed and standardized by members of the 3rd-Generation Partnership Project (3GPP) uplink (UL) communications (mobile-station-to-base-station transmissions) are generally based on the Single-Carrier Frequency-Division Multiple Access (SC-FDMA) scheme. SC-FDMA combines the desirable characteristics of Orthogonal Frequency Division Multiplexing (OFDM) with a relatively low Peak-to-Average Power Ratio (PAPR), which helps avoid excessive cost and power consumption of the mobile station, or “user equipment” (UE) in 3GPP terminology.
Just as in the LTE downlink (base-station-to-UE transmissions), LTE uplink transmissions incorporate two types of reference signals (RSs) to allow the base station (known as an “evolved Node B” or “eNB,” in 3GPP terminology) to perform coherent data demodulation and channel sounding. These reference signals are referred to as demodulation reference signals (DMRS) and sounding reference signals (SRS).
So that the important single-carrier property for LTE uplink transmissions can be maintained, the DMRS transmitted by any given UE occupy the same bandwidth as the UE's user data or control channel transmissions. Generally, these data and control channel transmissions, accompanied by the DMRS, are transmitted using the Physical Uplink Shared Channel (PUSCH) and Physical Uplink Control Channel (PUCCH), respectively. In the case of PUSCH, the UE simply uses one OFDM symbol per slot for DMRS transmission, as shown in FIG. 1. FIG. 1 illustrates a 1-millisecond uplink subframe, which includes two slots that in turn each include seven OFDM symbols (assuming a normal cyclic prefix). The DMRS is sent in the fourth symbol of each slot. In the frequency domain, the transmitted signal spans N resource blocks, where each resource block spans 12 subcarriers at a 25-kilohertz spacing and where N depends on an uplink transmission grant sent to the UE by the serving eNB. In the case of PUCCH, the UE transmits and time multiplexes multiple PUCCH-RSs within each subframe, spanning the PUCCH bandwidth assigned to the UE.
DMRS from different UEs within the same cell potentially interfere with each other and (given a synchronized network) with DMRS transmitted by UEs in neighboring cells. To limit the level of interference between the DMRS from different UEs, several techniques that facilitate the use of orthogonal or semi-orthogonal DMRS have been introduced to the LTE specifications. Thus, it can be generally assumed that the DMRS for each of several UEs within a given cell are orthogonal to one another, while the DMRS transmitted by UEs in neighboring cells are semi-orthogonal to those in the given cell. It will be appreciated, however, that UEs compliant to Release 11 of the 3GPP specifications for LTE support techniques that provide for orthogonality between DMRS transmitted by UEs belonging to different cells.
According to the 3GPP specifications for LTE, each DMRS comprises a pseudo-random signal generated in the frequency domain, and enjoys some special properties that make it suitable for channel estimation. A Base Sequence Index (BSI), Cyclic Shift (CS), and possibly an Orthogonal Cover Code (OCC) are combined to determine the transmitted signal corresponding to each DMRS. The following provides more details.
A group index and a sequence index together define the so-called BSI. As of Release 11 of the LTE specifications, BSIs are assigned in a UE-specific fashion. Different base sequences are semi-orthogonal, which implies that some inter-sequence interference is typically present if no additional measures are taken to ensure orthogonality. The DMRS for a given UE is transmitted over the same bandwidth occupied by the corresponding data signal (e.g., PUCCH, PUCCH), and the base sequence is correspondingly generated so that the DMRS signal is a function of the bandwidth.
To minimize the impact of interference peaks on DMRS, LTE introduces interference randomization techniques. In particular, sequence hopping and group hopping (jointly referred to as SGH) are BSI randomization techniques that operate on a slot level. SGH can be enabled and disabled on a per-cell basis by the use of cell-specific parameters broadcast by the eNB. These parameters, referred to in 3GPP specifications as “Group-hopping-enabled” and “Sequence-hopping-enabled,” affect group hopping and sequence hopping, respectively. For UEs compliant with at least Release 10 of the LTE specifications, SGH can be disabled in a UE-specific fashion by setting the UE-specific Radio Resource Control (RRC) parameter referred to as “Disable-sequence-group-hopping.”
Additionally, cyclic shift hopping (CSH) patterns provide further DMRS interference randomization by applying a UE-specific pseudo-random cyclic shift (CS) on a slot-by-slot basis. A CSH pattern dictates the different CS offsets applied in each of the slots; this CSH pattern is known to both the UE and eNB, so that it can be compensated for during channel estimation at the receiver end of the link.
Cyclic shifts (CS) comprise linear phase shifts applied to each BSI in the frequency domain. Orthogonal cover codes (OCC) comprise orthogonal time domain codes, operating on the DMRS provided for each UL subframe. In principle, OCC can be applied to an arbitrary number of DMRS. Orthogonal DMRS between UEs can be achieved by using CS, if the UEs have the same bandwidth and BSI, and by using OCC if the UEs do not employ sequence group hopping (SGH) and instead employ the same cyclic shift hopping (CSH) pattern.
CS comprises a method to achieve orthogonality based on cyclic time shifts, under certain propagation conditions, among DMRSs generated from the same base sequence. Only eight different CS values can be dynamically indexed in Rel-8/9/10, even though in practice fewer than eight orthogonal DMRS can be achieved, depending on channel propagation properties (without considering OCC in this example). Even though CS is effective in multiplexing DMRSs assigned to fully overlapping bandwidths, orthogonality is lost when the bandwidths differ and/or when the interfering UE employs another base sequence or CSH pattern.
The OCC code [+1, −1] is able to suppress an interfering DMRS as long as the reference signal's contribution after the matched filter at the receiver is identical for both DMRSs of the same subframe. Similarly, the OCC code [+1, +1] is able to suppress an interfering DMRS as long as its contribution after the eNB matched filter has opposite sign respectively on the two DMRSs of the same subframe.
While base-sequences are assigned in a semi-static fashion, CS and OCC are dynamically assigned as part of the scheduling grant for each UL PUSCH transmission, and thus for PUSCH DMRS. The CS/OCC assignment method for PUCCH DMRS is different.
While different implementations are possible, a typical channel estimator performs a matched filter operation of the received signal corresponding to each DMRS with the known transmitted DMRS. The matched filter operation can be equivalently performed in time or frequency domains. If OCC is applied, the multiple DMRSs spanning the OCC code are combined according to the corresponding OCC.
Given the exploding demand for wireless data services, it is anticipated that the deployment of so-called “small cells” will be an important approach to improve network capabilities and to ensure seamless coverage. In some small cell scenarios, e.g., indoor and hotspot scenarios, the following characteristics are observed: small delay spread, low mobility, and a small number of users. These characteristics result in a relatively stationary channel in the time and frequency domains as well as relatively low Doppler shifts for small cells. Another characteristic is that it is not unusual to have a less-than-ideal backhaul connecting small cells to the rest of the network, with latency values up to 50 milliseconds. This results in slow coordination among small cells (and macro cells). Even with small cell scenarios, however, there remains a need for improved performance, e.g., improved spectral efficiency. Accordingly, it should be considered whether the currently defined approaches to the handling of DMRS and other reference signals can be improved, to better support the unique demands of small cells.